River processes
Erosion
Erosion is the wearing away and removal of rocks and sediments by a moving force. The energy of a river flowing downhill causes erosion. Rivers can erode laterally (sideways), vertically (downcutting) or back into the upland area (headward), all of which result in the river becoming, longer, deeper and wider.
Erosion is the wearing away and removal of rocks and sediments by a moving force. The energy of a river flowing downhill causes erosion. Rivers can erode laterally (sideways), vertically (downcutting) or back into the upland area (headward), all of which result in the river becoming, longer, deeper and wider.
Headward Erosion
Headward erosion takes place close to the source of the river as throughflow and surface runoff. It causes erosion at the valley head (the point the water enters the river channel), and increases the length of the river channel. Vertical Erosion Vertical erosion deepens the river channel and takes place at the upper stages of the river. Lateral Erosion Lateral erosion increases the width of the river channel and takes place in the middle and lower stages of a river. |
The five main processes in which river erosion occurs
Hydraulic action - the pressure of the water breaks rock and soil particles away from the river bed and bank. Hydraulic action is strongest where water movement is most intense such as at waterfalls, and during floods.
Abrasion (corrosion) - pieces of eroded rock rub and scrape along the river bed and banks which leads to the removal of material. Abrasion is responsible for the biggest proportion of erosion to river beds and banks.
Attrition - this is when eroded rocks smash into each other, this leads to rock particles being broken down into smaller pieces and also to them becoming more rounded as they rub together. Attrition doesn't erode the river bed or banks it just helps to breakdown particles which are carried within the river channel.
Cavitation - in turbulent (erratic swirling water) stretches of water, air bubbles implode this causes shockwaves which erode the soil and rocks that line the river bed and banks.
Corrosion (solution) - this is the dissolving of rock by chemical processes. When Carbon Dioxide dissolves in water it forms carbonic acid, this reacts with rocks such as limestone and chalk and breaks them down.
Hydraulic action - the pressure of the water breaks rock and soil particles away from the river bed and bank. Hydraulic action is strongest where water movement is most intense such as at waterfalls, and during floods.
Abrasion (corrosion) - pieces of eroded rock rub and scrape along the river bed and banks which leads to the removal of material. Abrasion is responsible for the biggest proportion of erosion to river beds and banks.
Attrition - this is when eroded rocks smash into each other, this leads to rock particles being broken down into smaller pieces and also to them becoming more rounded as they rub together. Attrition doesn't erode the river bed or banks it just helps to breakdown particles which are carried within the river channel.
Cavitation - in turbulent (erratic swirling water) stretches of water, air bubbles implode this causes shockwaves which erode the soil and rocks that line the river bed and banks.
Corrosion (solution) - this is the dissolving of rock by chemical processes. When Carbon Dioxide dissolves in water it forms carbonic acid, this reacts with rocks such as limestone and chalk and breaks them down.
Transportation
Transportation is the process by which eroded material is carried in a river channel.
The energy to transport the river's load comes from its velocity.
Eroded material carried within a river is called its load.
Transportation is the process by which eroded material is carried in a river channel.
The energy to transport the river's load comes from its velocity.
Eroded material carried within a river is called its load.
The four ways which eroded material is transported
Suspension - turbulence draws very fine particles such as clays and silts up into the water which are then carried along by the flow of the river. Traction - the force of the water pushes very large particles such as boulders along the river bed. Solution - dissolved substances such as limestone, are carried along in the water. Saltation - Larger particles like pebbles and gravel are too heavy to be carried along suspended in the water. Particles such as these bounced along the river bed by the force of the water. Materials that are carried along by saltation and traction are referred to as the river's bedload. |
Deposition - the process of dropping eroded material
When a river slows down it loses energy which results in it dropping some of its load. This process is called deposition.
Ways in which a river can lose speed and energy:
When a river slows down it loses energy which results in it dropping some of its load. This process is called deposition.
Ways in which a river can lose speed and energy:
- reduced rainfall - leads to reduced discharge, which results in the river slowing down and having less energy
- increased evaporation or abstraction - reduces discharge, slows the river and reduces the rivers energy
- friction - Shallow areas and areas close to the bank, reduce the river's speed and its energy
- anywhere the river is forced to slow down it loses energy, for example, just before a narrow section
- when a river meets the sea it loses a lot of energy which is absorbed by the sea.
Capacity - total amount of material a river can carry
A river's capacity is the total load it can transport at a given point.
Total load is measured in volume, weight and mass.
The river's load is divided into different categories which are determined by particle size.
Particles sizes range from silt and clay (less than 0.1 mm in diameter) to large boulders.
Competence is the maximum particle size that a river can transport at a given point.
A river's capacity is the total load it can transport at a given point.
Total load is measured in volume, weight and mass.
The river's load is divided into different categories which are determined by particle size.
Particles sizes range from silt and clay (less than 0.1 mm in diameter) to large boulders.
Competence is the maximum particle size that a river can transport at a given point.
The Hjulstrom curve shows the link between river velocity and competence. The competence of a river is affected by the amount of energy it has, which is affected by it's velocity. As a general rule the greater a river's velocity, the more energy it has. The Hjulstrom curve also shows how the processes of erosion, deposition and transportation vary with velocity.
The mean or critical erosion velocity curve shows the minimum velocity necessary to erode, pick up and transport different size particles, either as bedload or in suspension. The mean fall or setting velocity curve show the competence of the river at different velocities (velocities at which different sized particles are deposited). The graph shows that small particles of sand (0.1 to 1 mm diameter) can be eroded at lower velocities than silts and clays. Clays and silts stick together a lot easer than sand particles, because of this silts and clayss require a greater velocity (more energy) to dislodge and erode them than sands do. |
sources
AQA (2012): AS & A2 Geography complete revision and practice all in one book
BBC Bitesize. I-sda.eu
AQA (2012): AS & A2 Geography complete revision and practice all in one book
BBC Bitesize. I-sda.eu